This invention relates to a cathode-ray tube such as a color television picture tube. More particularly, it relates to a cathode-ray tube with an anti-reflective coating and a method of forming the anti-reflective coating.
It is known that the contrast performance of a cathode-ray tube is improved by reducing the optical transmittance of its faceplate. The demand for high image quality has led to the replacement of formerly-common clear faceplates having a transmittance of about eighty-five percent and gray faceplates having a transmittance of about sixty-nine percent by tinted faceplates having a transmittance of about fifty percent and dark-tinted faceplates having a transmittance of only about thirty-eight percent. To counter the attendant loss of brightness, and to improve focusing performance and permit larger screen dimensions, recent cathode-ray tubes also employ high accelerating voltages. Two resulting problems are specular reflection and charge-up.
Specular reflection refers to mirror-like reflection of ambient light from the outer surface of the faceplate. In clear and gray faceplates such specular reflection is generally masked by diffuse reflection from the inner surface of the faceplate, but in tinted and dark-tinted faceplates diffuse reflection is reduced and specular reflection becomes more noticeable. As a form of glare, specular reflection is a source of eye fatigue, and it is annoying for the viewer to see reflections of external objects (such as the viewer's own face) superimposed on the intended image.
Charge-up refers to the charging of the faceplate to a strong positive or negative potential when the cathode-ray tube is switched on or off, as a consequence of the high accelerating voltage. Undesirable results include crackling sounds, electrical discharges between the faceplate and the human body, and attraction of particles of dust and dirt to the faceplate.
The faceplates of some recent cathode-ray tubes have a silica coating with an inclusion of conductive filler particles and a dye or pigment. The conductive filler greatly reduces charge-up. The dye or pigment selectively absorbs light, thereby further reducing the optical transmittance of the faceplate and improving its contrast performance. The reduced transmittance, however, aggravates the problem of specular reflection. Specular reflection becomes particularly objectionable when the above type of coating is applied to a faceplate having a transmittance of fifty percent or less.
Past attempts to reduce specular reflection include roughening the surface of the faceplate, and providing an anti-reflective interference coating comprising, for example, layers of titanium oxide and magnesium fluoride. Roughening the faceplate, however, involves a loss of structural strength and image definition. Interference coatings are attractive, but they have conventionally been formed by vacuum processes such as evaporation deposition, the high cost of which has limited interference coatings to special-purpose cathode-ray tubes and ruled out their use in consumer items such as color television sets.